POLYACETAL RESIN COMPOSITION

Abstract: The present invention aims to provide a polyacetal resin composition having excellent friction-wear resistance characteristics, said composition containing not more than 15 % by weight of a polyolefin resin that has a density of not more than 0.93 g/cm<3> and a weight average molecular weight of less than 500,000.

Description: TITLE POLYACETAL RESIN COMPOSITION DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a polyacetal resin composition, the principle component of which is a polyacetal resin, and to a resin composition that has excellent friction-wear characteristics suitable for use in sliding parts. It further specifically relates to a polyacetal resin composition that provides good friction-wear resistance when used as a sliding part against a counter sliding part regardless of its material of construction. Polyacetal resins, which are thermoplastic, exhibit excellent mechanical properties and heat resistance, particularly good friction-wear resistance characteristics and are, thus, used in sliding parts applications in a broad range of fields. In order to further improve upon the friction-wear characteristics of polyacetal resins, methods are known for incorporating inorganic solid lubricants, such as molybdenum disulfide and graphite, petroleum-type lubricants, synthetic lubricants, aliphatic alcohol or its ester-type liquid or semi-liquid lubricants, or a polyolefin resin fine powder having a molecular weight of at least 500,000. However, the incorporation of a lubricant in a polyacetal resin for the objective of improving its friction-wear characteristics has also been known to cause problems, such as a reduction in the physical properties inherent in the resin or the slippage of pellets on the screw surfaces or cylinder inner surfaces in a screw reversing cycle where the resin is processed by an injection molder or extruder, resulting in a failure of the resin to be grabbed by the molding machine, with a resultant considerable decrease in productivity. Another problem is that there has been a substantial change in the friction-wear friction resistance, which is greatly dependent upon the material of the counter sliding part that comes in contact with the resin, with the results that good behavior is shown with a certain counter part material, but poor wear behavior is exhibited toward another material. Due to of the trend of the friction-wear characteristics to vary depending upon the type of material used for the part which comes in contact with the polyacetal molded article, one has had to select a lubricant, taking into consideration the type of material used in the sliding part that comes in contact therewith, so as to achieve the desired friction wear- characteristics. There is, accordingly, a need for a polyacetal resin with good friction-wear resistance regardless of the counter part material. Extensive studies were carried out on a polyacetal resin composition that provides a stable friction-wear characteristic regardless of the difference in the material of the sliding material which comes in contact therewith, even under high speed, high load sliding conditions, without losing the excellent polyacetal resin properties, which led to the finding that an effective method is to incorporate not more than 15% by weight of a polyolefin resin that has a density of not higher than 0.93 g/cm 3 and a weight average molecular weight of not more than 500,000. Thus, the present invention relates to a polyacetal resin composition comprising not more than 15% by weight of a polyolefin resin, preferably at least 0.5% by weight up to 15% by weight, that has a density of not more than 0.93 g/cm 3 and a weight average molecular weight of less than 500,000. By the term "polyacetal resin", is meant not only a polyacetal homopolymer, but also a polyacetal copolymer with a cyclic either or a cyclic polymer. It is, however, preferred to use a polyacetal homopolymer because the addition of a lubricant to a copolymer sometimes will reduce its physical properties (tensile characteristics, Izod impact characteristics, flexural characteristics, compression properties, and the like). The term polyolefin resin, as used in this invention, refers to aliphatic-alpha-olefin homopolymers, such as polyethylene, polypropylene, and polymethyl pentene, or copolymers of aliphatic-alpha-olefins themselves. Such copolymers may contain a small amount of a diene-type hydrocarbon comonomer. The polyolefin resin used in this invention must have a density of not higher than 0.93 g/cm 3 and a weight average molecular weight of not higher than 500,000. Polyolefins having such densities and molecular weights disperse in the polyacetal in fine particles of not more than 10 microns in size. Preferred polyolefin resins which are commercially readily available are polyethylene and polypropylene, most preferably polyethylene. The polyacetal resin composition of this invention is obtained by mixing at least 85% by weight of a polyacetal resin and up to 15%, inclusive, by weight of a polyolefin resin in a conventional mixer. The polyolefin resin should be used normally in an amount of at least 0.5% by weight, preferably in a range of 0.5-10% by weight. The use of more than 15% by weight will reduce the excellent physical properties inherent in the polyacetal resin and also will deteriorate its processability. The polyacetal resin composition of this invention can be used with no differences in the areas in which the polyacetal resins have been previously used. Use of this composition with a sliding part material is particularly preferred because it exhibits high frictional wear resistance, regardless of the material of the counter sliding part material that it comes in contact therewith. The polyacetal resin composition of this invention may be further compounded with various additives that are used in polyacetal resins, such as a stabilizer, a nucleating agent, an antistatic agent, a flame retardant, a colorant, a lubricant, or the like, as well as with fibrous or granular fillers or reinforcing agents. The outstanding wear characteristics of the polyacetal resin composition of this invention are specifically shown below. The following materials were used: Polyacetal Resin Composition (Sample 1) Polyacetal Resin A. Polyacetal Homopolymer, Delrin ® 900NC (a product of E. I. du Pont de Nemours and Company). Resins Incorporated (Polyethylene Resins and Other Resins^ HDPE = High-density polyethylene, HD 6733.19 (a product of the Exxon Company) Density: 0.95 g/cm 3 LLDPE = Linear low-density polyethylene, LL 6101.59 (a product of the Exxon Company) Density: 0.92 g/cm 3 LDPE = Low-density polyethylene, KN 230 (a product of the Chevron Chemical Company) Density: 0.92 g/cm 3 EPDM = Ethylene propylene diene copolymer, Nordel 3681 (a product of E. I. du Pont de Nemours and Company) Density: 0.86 g/cm 3 EVA = Ethylene-vinyl acetate copolymer, ELVAX 40 ( a * • ■ •■ ■ product of E. I. du Pont de Nemours and Company) Density: 0.95 g/cm 3 PE-g-As = Polyethylene-styrene/acrylonitrile grafted copolymer, Modiper 1400 (a product of Nippon Yushi) Density: 0.97 g/cm 3 The specimens of Test 1 each contained 2.0% of a resin additive comprising a polyolefin and other resin. Material Used for Sliding Counterpart (Sample 2. Polyacetal Resin Polyacetal copolymer, Duracon M9044 ( a product of Polyplastics). ABS = ABS Resin, Cycolac 5500 (a product of the GE Company). PET = 20% glass fiber reinforced polyethylene terephthalate, Rynite ® 530 (a product of E. I. du Pont de Nemours and Company). Test Procedure A polyacetal resin composition sheet (Sample 1) is placed on a tabletop capable of making a horizontal reciprocal movement with an amplitude of 2.75 inches; on the polyacetal resin sheet top surface is placed a 1/4 inch wide by 1 inch long sliding counter material-made from a resin sheet (Sample 2), fixed so as not to make any horizontal movement, and a 40 psi load is applied to Sample 2. The tabletop was allowed to make a typical reciprocal movement at an average rate of 30 feet/min for 3 hours. The weight loss of each resin sheet before and after the above test was measured and wear characteristics was calculated by the following equation: Equation 1 W =K x F x Vx T W = wear volume (inch 3 ) = Wei g ht Loss of Sam p le 1 + Wei g ht Loss of Sample 2 Average Density of Samples 1 and 2 K = coefficient of wear (inch 3 /min/ft. lb. hr) F = force (lb) V = rate (ft/min) T = time (hr) The test results are given in the following Table. Test Nos. 1-4 are Examples of this invention and Test Nos. 5-6 are Comparative Examples. It should be understood from these results that the samples comprising the composition of this invention exhibit nearly constant wear characteristics regardless of the material used for the counterpart material which comes in contact therewith. Table 1 The polyacetal resin composition of this invention constantly gives excellent friction-wear resistance characteristics regardless of the sliding conditions, such as load and speed, or regardless of the material of the counter surface. In particular, the polyacetal resin composition of this invention can exhibit excellent sliding characteristics of friction-wear resistance with low coefficients of friction regardless of the sliding part counter part material even when the sliding surfaces have elevated temperatures, generated by the heat of friction.

Claims: CLAIMS 1. A polyacetal resin composition comprising up to 15% by weight, inclusive, of a polyolefin resin having a density of not more than 0.93 g/cm 3 and a weight average molecular weight of less than 500,000. 2. The composition of Claim 1 wherein the polyacetal is a homopolymer. 3. The composition of Claim 1 wherein the polyacetal is a copolymer.

Inventors: SHINOHARA KENICHI, TAKAHASHI TATSUHIRO, SHINOHARA, KENICHI, TAKAHASHI, TATSUHIRO
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1994
Main classification: C08L59
IPC codes: C08L23, C08L55, C08L59, C08L67, F16C33